Saitec Offshore Technologies has developed the SATH (Swing Around Twin Hull) floating wind turbine solution. SATH is a full-made of pre-stressed concrete floating platform consisting of two cylindrical and horizontal hulls with conical ends linked to each other through bar frames. The platform is moored by a Single Point Mooring (SPM) that allows SATH to rotate around as weathervane, helping yaw control system to maintain the rotor plane facing the wind. The first full scale prototype of floating wind turbine (2.0MW) using SATH technology will be installed in BIMEP (Biscay Marine Energy Platform). The project has started early in 2016 and comprises the engineering phase, construction, assembly and installation of the whole system and 2 years of operation, monitoring and energy production.The final scope of this first demonstration project is to technically prove the feasibility of the solution and its ability to withstand the harsh Atlantic environment with no effects in the energy production. To reach this objective, an intensive test campaign using a scale model has been performed during the first phase of the project. This study aims to describe the methodology followed to perform the basin test as well as to process the experimental data for enhancing the final design of the full-scale prototype and its monitoring system.

Method

The 1:36 scale model used for the test campaign conducted in the Cantabria Coastal & Ocean Basin (CCOB) has been designed to be a realistic representation of the full-scale device. It consists of a steel platform and tower with a detailed scaled mooring system. All the representative variables involved in the testing have been properly scaled using Froude similitude. The wind turbine has been modeled with a set of fans that represents both the total wind thrust of the rotor and the effects of the gyroscopic momentum. The platform has been exposed to different combinations of wind and wave cases with different incident angles. The data acquired during the experiments has been processed and used to calibrate a fully-coupled numerical model.

Results

The main outcome of this study has been a calibrated numerical model that accounts for the first and second order nonlinearities of the system, shown specially at high period ranges in the experiments. The originally developed linear numerical model was found to be excessively conservative specially close to the resonance zone of the structure. The calibration of the model through the quadratic damping matrix has allowed for a realistic representation of the response of the full-scale SATH prototype. Additionally, experimental testing has proved the feasibility of the proposed solution exposed to the specific site conditions of BIMEP. During testing, the critical parameters have been also targeted in order to obtain the optimal configuration of the monitoring system that will be installed in the full-scale FWT.

Conclusions

Wave tank testing has proved the safe operation of the full-scale SATH prototype under normal and extreme conditions. The decision of the monitoring system installation in the prototype will be based on an analysis of the impact level of each of the variables involved in the problem using the data obtained from the test campaign. A proper structural monitoring system will be helpful for ensuring the health of the structure, reducing the risk and extending the lifetime. Moreover, this data could be used for further validation of the numerical model, which would be able to simulate the real response of the floating wind turbine under any specific conditions.

Objectives

This work has explained step by step the design of a floating wind turbine during the engineering phase. The challenges found during the project, as well as the given solutions to overcome them have been addressed. In addition, it has been shown the importance of a proper installation of a monitoring system targeting the critical variables in offshore structures. Finally, it has been demonstrated that SATH, which is a different solution from conventional Oil and Gas technologies, is fully suitable as a floating offshore wind substructure.